Switching converters are widely used due to their high efficiency and simple internal structure. Many control modes could be used to control switching converters, such as constant on time (COT) control mode, peak current control mode and average current control mode. Among these control modes, COT control mode is getting more and more popular as its fast transient response, simple structure and smooth switch of operation mode.
FIG. 1 schematically illustrates a prior art switching converter 1. The switching converter 10 comprises a control circuit 100 and a switching circuit 110. The switching circuit 110 adopts a synchronous buck topology, comprising a high side switch M1, a low side switch M2, an inductor L and an output capacitor C. The high side switch M1 has a first terminal configured to receive an input voltage VIN, a control terminal configured to receive a switching signal CTRL, and a second terminal. The low side switch has a first terminal coupled to the second terminal of the high side switch M1, a control terminal and a second terminal connected to ground. The inductor L has a first terminal connected to the second terminal of the high side switch M1 and a second terminal configured to provide an output voltage VOUT. The switching circuit 410 converts the input voltage VIN to the output voltage VOUT under the control of the switching signal CTRL.
The control circuit 100 comprises an ON signal generating circuit 101, an OFF signal generating circuit 102, a logic circuit 103 and a zero current protection comparator 104. The ON signal generating circuit 101 has a first input terminal, a second input terminal and an output terminal. Based on a comparing result between a reference signal VREF received at the first input terminal and the output voltage VOUT received at the second input terminal, the ON signal generating circuit 101 provides an ON signal SETON at the output terminal. The OFF signal generating circuit 102 provides an OFF signal SETOFF at the output terminal. The OFF signal generating circuit 102 begins timing when the high side switch M1 is turned on, and the OFF signal SETOFF jumps from a first status (e.g. logic low) to a second status (e.g. logic high) when a preset time TS ends. The logic circuit 103 has a first input terminal, a second input terminal and an output terminal. The logic circuit 103 provides the switching signal CTRL at the output terminal based on the ON signal SETON received at the first input terminal and the OFF signal SETOFF received at the second input terminal. Normally, the high side switch M1 is turned on when the ON signal SETON is enabled (e.g. jumping from a first status to a second status). The high side switch M1 is turned off when the OFF signal SETOFF is enabled (e.g. jumping from a first status to a second status).
Under light load conditions, the inductor current IL may fall below zero. As a result, a current would flow from the output capacitor C through the inductor L and the low side switch M2 into ground, thereby affecting the efficiency of the switching converter 10. So, the control circuit 100 further comprises a zero current protection comparator 104. The zero current protection comparator 104 compares a current through the low side switch M2 (low side switch current IM2) with a current threshold IZE, configured to turn off the low side switch M2 when the low side switch current IM2 falls down around the current threshold IZE, so as to avoid a large negative current through the low side switch M2. Setting the current threshold IZE is a difficult work. A small current threshold IZE may lead to a large negative current through the low side switch caused by time delay of the comparator 104 or rapid change of the current through the low side switch M2. A large current threshold IZE may lead to false triggering or no-triggering of the protection function of the comparator 104. For example, supposing that the current threshold IZE is set to around 200 mA, the threshold may be higher than the peak value of the inductor current IL under light load conditions. So, after the high side switch M1 is turned off, the zero current protection comparator 104 would turn off the low side switch M2 directly since the current through the low side switch is smaller than the current threshold IZE.
Designing control circuit with zero current protection function becomes a big challenge.